1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for controlling a transmit antenna array (TxAA) for a physical downlink shared channel (PDSCH) in a soft handover (SHO) region.
2. Description of the Related Art
In general, a physical downlink shared channel (PDSCH) used in an asynchronous W-CDMA (Wideband Code Division Multiple Access) mobile communication system, a type of a 3rd generation mobile communication system, refers a channel shared by a plurality of UEs (User Equipments). The PDSCH is a channel assigned to transmit packet data or high-rate data to the UEs in a 10 ms radio frame unit, and is commonly used by a plurality of the UEs. The PDSCH can vary a data rate of transmission data in a frame unit, and can also perform weight adjustment on a transmit antenna array and power control in a slot unit, like a dedicated channel (DCH) established between a Node B and a UE in the W-CDMA system. The radio frame, a basic unit for transmitting signals in the W-CDMA system, has a length of 10 ms, and each radio frame is comprised of 15 slots. In addition, the PDSCH is a channel for transmitting user data only. When the PDSCH is assigned to the UE for power control on the PDSCH, a downlink dedicated physical channel (DL-DPCH) is assigned to the UE in association with the PDSCH. Here, the DL-DPCH becomes a channel for power control on the PDSCH. The PDSCH can be continuously transmitted to one UE over a plurality of frames. Alternatively, the PDSCH can be transmitted to the UE over only one frame. In addition, the time to transmit the frames to a plurality of the UEs is determined by scheduling in an upper layer.
A structure of the PDSCH and a structure of the DL-DPCH assigned to the UE in association with the PDSCH will be described with FIGS. 1A and 1B. FIG. 1A illustrates a structure of the PDSCH used in a mobile communication system, and FIG. 1B illustrates a structure of the DL-DPCH assigned to the UE in association with the PDSCH.
Referring to FIG. 1A, a radio frame 101 of the PDSCH is 10 ms in length, and is comprised of 15 slots Slot#0–Slot#14. Each slot, e.g., a slot Slot#i (103), has a length of 2560 chips, and an amount of information transmitted over the Slot#i 103 is in inverse proportion to a spreading factor (SF) used for the PDSCH slots. The SF has a value of 4 to 256, and transmission information data is spread according to the SF value. Only the user data is transmitted over the PDSCH.
Next, referring to FIG. 1B, when the PDSCH shown in FIG. 1A is used, the DL-DPCH associated with the PDSCH is assigned to the corresponding UE by the Node B. A radio frame 111 of the DL-DPCH is also comprised of 15 slots Slot#0–Slot#14, and each slot, as shown in FIG. 1B, is comprised of Data1 bits 112, TPC (Transmit Power Control) bits 113, TFCI (Transmit Format Combination Indicator) bits 114, Data2 bits 115, and Pilot bits 116. Each slot of the DL-DPCH can have various structures according to the lengths of Data1, TPC, TFCI, Data2 and Pilot.
The Data1 bits 112 and the Data2 bits 115 are transmitted over a downlink dedicated physical data channel (DL-DPDCH), and the DL-DPDCH transmits user data and signaling information from the upper layer. The TPC 113, the TFCI 114, and the Pilot 116 are transmitted over a downlink dedicated physical control channel (DL-DPCCH). The TPC 113 is a field for transmitting a command for controlling transmission power of uplink channels transmitted from the UE to the Node B; the TFCI 114 is a field for transmitting a codeword indicating that transport channels having different data rates are transmitted over the DL-DPCH; and the Pilot 116 is a field for enabling the UE to measure transmission power of a downlink signal for power control on the received downlink signal. Here, the “transport channel” refers to a channel serving to connect a physical channel for actually transmitting the data to the upper layer.
In the W-CDMA system, for a closed-loop transmit antenna array for the PDSCH, the UE receiving the DL-DPCH 111 transmits, to the Node B, weight information acquired by measuring a common pilot channel (CPICH) received from the Node B. That is, upon receiving the CPICH transmitted from the Node B, the UE compensates for its phase difference thereby to detect a weight proper for the maximum receiving power level. The weight information created depending on the detected weight is transmitted to the Node B, and the Node B applies corresponding weights to respective antennas for the DL-DPCH transmitted to the UE, depending on the weight information of the DL-DPCH and/or PDSCH received from the UE, before transmission.
With reference to FIG. 2, a description will be made of the downlink and uplink signal flows for the case where the UE receiving the PDSCH is located in a soft handover (or handoff) region. FIG. 2 illustrates downlink and uplink signal flows for the case where a UE receiving a PDSCH is located in a soft handover region, wherein for simplicity, only two Node Bs are considered.
In a soft handover (SHO) process, when a UE moves away from a current Node B1 in communication with the UE, and at the same time moves to an area where it can receive signals from an adjacent new Node B2, the UE receives the signals not only from the current Node B1 but also from the new Node B2. In this state, if a quality (or level) of the signal received from the Node B1 is less than a predetermined threshold, the UE releases the channel established to the Node B1, and then establishes a new channel to the Node B2 providing high-quality signals, thus performing the handover process. By doing so, it is possible to maintain a call without interruption.
Referring to FIG. 2, a Node B1 201, which is currently communicated with a UE 211, transmits a PDSCH and a DL-DPCH associated with the PDSCH to the UE 211. However, a Node B2 203 transmits only the DL-DPCH to the UE 211 when the UE 211 moves to an SHO region between the Node B1 201 and the Node B2 203. A set of all the Node Bs set to transmit signals to the UE 211 existing in the SHO region is called an “active set”. That is, the Node B1 201 transmits both the DL-DPCH and the PDSCH to the UE 211, and Node B2 203 is newly admitted to the active set and transmits only a DL-DPCH to the UE 211. The UE 211 broadcasts the UL-DPCH to Node B1 201 and Node B2 203 indiscriminately. In the prior art, when the UE 211 enters an SHO region, the UE 211 receives CPICHs from node B1 201 and node B2 203 together and measures the signal strengths of the CPICHs to select a primary node B among the Node Bs. The UE 211 transmits the temporary ID of a node B designated as a primary Node B in the feedback information (FBI) field of a UL_DCH. The FBI is 2 fields in length as shown. An S field of the FBI that the UE 211 transmits to the node B when SSDT (Site Selection Diversity Transmission) is employed. A D field of the FBI that the UE 211 transmits to a Node B when transmission antenna diversity is employed. The S field consists of 0, 1 or 2 bits. If the S field consists of 0 bit, this implies that the SSDT is not used. If the SSDT is used, the FBI field transmits a codeword representing the temporary ID of a primary Node B. The D field consists of 0, 1, or 2 bits. If the D field consists of 0 bit, this implies that the transmission antenna diversity is not used. In the case of 1 bit, the transmission antenna diversity is used together with the SSDT, and in the case of 2 bits, only the transmission antenna diversity is adopted.
When the UE 211 receiving the PDSCH from the Node B1 201 exists in the SHO region, a problem occurs in that the UE 211 receives both the PDSCH and the DL-DPCH from the Node B1 201 but receives only the DL-DPCH from the Node B2 203. Here, the typical reason that the PDSCH does not support the SHO is because compared with the DL-DPCH, the PDSCH transmits data at a relatively high data rate, thus consuming an increased number of channel resources of the Node B. As a result, system capacity is affected. In addition, the W-CDMA mobile communication system may have a timing problem due to non-synchronization between the Node Bs. In order to support the SHO, the PDSCH shared by a plurality of the UEs requires elaborate scheduling for the time points where it is used by the respective UEs. In light of the scheduling, it is difficult to embody transmission of the PDSCH from the new Node B to the UE.
The DL-DPCHs transmitted from the Node B1 201 and the Node B2 203 are received at the UE 211, and then subjected to soft combining. Here, “soft combining” refers to combining the signals received at the UE through different paths. Therefore, by calculating a phase difference between the CPICHs received from the Node Bs and then compensating for the phase difference, it is possible to reduce the influence of fading and noise, which affect the signals received at the UE 211. Soft combining is available only when the UE 211 receives the same information from the different Node Bs. However, when the UE 211 receives different information from the Node Bs, the received information, though subjected to soft combining, will be recognized as a noise component, resulting in an increase in the noise component of the signal.
In the process of analyzing the DL-DPCH, the downlink signals transmitted to the UE 211 from the respective Node Bs, i.e., the Node B1 201 and the Node B2 203, are subjected to soft combining except for the TPC bits 113 shown in FIG. 1B. The reason that the TPC 113 is analyzed separately rather than being analyzed by soft combining is because the TPCs received at the UE 211 from the respective Node Bs may be different from each other, since the signal received at the Node B1 201 from the UE 211 is high in level while the signal received at the Node B2 203 from the UE 211 is low in level, or vice versa, due to movement of the UE 211. Therefore, the TPC 113 is analyzed through a separate TPC analysis algorithm for a plurality of the Node Bs, rather than being subjected to soft combining.
With reference to FIG. 2, a description has been made of the downlink and the uplink signals for the case where the UE is located in the SHO region. Next, an operation of a transmit antenna array (TxAA) supporting the SHO will be described with reference to FIG. 3.
FIG. 3 illustrates an operation of a transmit antenna array using the conventional soft handover scheme. Referring to FIG. 3, when a UE 311 is located in an SHO region during a call, a Node B1 301 and a Node B2 303 transmitting signals to the UE 311 decrease their transmission power, for soft handover of the call service, and at the same time, the UE 311 calculates weights depending on a phase difference between the CPICHs transmitted from both of the Node Bs so as to maximize SINR (Signal-to-Interference+Noise Ratio), and then feeds the weights back to the Node B1 301 and the new Node B2 303 over the DL-DPCH and the PDSCH of the Node B1 301 and the new Node B2 303.
The UE 311 soft-combines the signals received from the Node B1 301 and the Node B2 303 with the weights transmitted to the Node Bs over a FBI (Feedback Information) field of an uplink dedicated physical control channel (UL-DPCCH) shown in FIG. 3, and then, determines the weights so as to maximize the SINR of the soft-combined received signals. That is, since the UE 311 soft-combines the signals received from the Node B1 301 and the Node B2 303 and determines the feedback weights of the received signals so as to maximize the SINR, a conventional method for applying an optimal weight for the PDSCH, in which only one Node B in the SHO region, e.g., the Node B1 301 having the highest received signal level should transmit the signals, has the following disadvantages:
If the UE 311 is located in a non-SHO region, the transmit antenna weights for the PDSCH and the DL-DPCH transmitted to the UE 311 are identical to each other. In other words, the transmit antenna array weight for the PDSCH is determined in association with the DL-DPCH. A change in a ratio of the phase and size of the two antennas for the DL-DPCH causes an equivalent change in the weights, since the PDSCH is transmitted over the same channel. For this reason, for the weight for the PDSCH, the weight for the associated DL-DPCH is used.
However, if the UE 311 is located in the SHO region, the transmit antenna arrays for the DL-DPCH and the PDSCH are equally determined using the weights determined by measuring a phase difference between the CPICHs from not only the Node B transmitting the PDSCH to the UE 311 but also other Node Bs registered in the active set. Specifically describing this with reference to FIG. 3, the transmit antenna array weight for the PDSCH is determined considering a channel environment between the UE 311 and the Node B having the highest received signal level, i.e., the Node B1 301, while the transmit antenna array weight for the DL-DPCH is determined considering the channel environment to not only the Node B1 301 having the highest received signal level but also the Node B2 303 in the active set of the UE 311.
As described above, since the transmit antenna weight for the PDSCH according to the conventional SHO scheme is proposed to have the same value as the transmit antenna weight for the DL-DPCH, the above problem occurs. That is, in the SHO region, the weights transmitted from the Node Bs to the UE 311 are determined considering not only the channel environment between the UE 311 and the Node B1 301 but also the channel environment between the UE 311 and the Node B2 303. Thus, if the intact weights are applied, the PDSCH will be provided with a weight difference from a weight for the actual transmit antenna array. Therefore, the method for applying the same weight to the transmit antenna arrays, as used for the DL-DPCH and the PDSCH in the non-SHO region, cannot be used in the SHO region. Accordingly, there has been a demand for an apparatus and method for properly controlling a transmit antenna array for the PDSCH in the SHO region.